Ehrlichia species are emerging zoonotic pathogens that cause acute systemic diseases (ehrlichiosis) in humans. The human granulocytic ehrlichiosis (HGE) agent infects granulocytes. E. chaffeenis and E. sennetsu infect monocytes/macrophages. Work in the P.I.'s laboratory indicates that the ehrlichiae bind by a protein ligand to a host surface protein receptor, induce protein tyrosine kinase-dependent internalization and multiply in the unique early endosome which selectively accumulates transferrin receptors (TfR). The mechanisms involved in the accumulation of TfR by the ehrlichial inclusions and how lysosomal fusion is prevented is unknown. The P.I. has developed three hypotheses to be tested. Hypothesis 1 is that the three ehrlichial species have a specific outer membrane protein which binds to the specific host cell receptor that would induce clathrin-dependent uptake and not activate the cells. Hypothesis 2 is that the interaction of the OMP and the host cell receptor induces the selective fusion of ehrlichial inclusions with TfR-containing endosomes. TfR is required to provide an essential iron to ehrlichia and prevent lysosomal fusion. Hypothesis 3 is that immunization of the host with the OMP protein can protect the host from the ehrlichiosis. To address these hypotheses, the P.I. proposes to accomplish five specific aims. Aim 1 will test hypothesis 1 by preparing the outer membrane fraction from purified HGE agent, E. chaffeensis and E. sennetsu, and genetically and antigenically characterizing selected OMP s that are likely to be involved in ehrlichial binding and internalization. Aim 2 will also test hypothesis 1 by examining whether the three Ehrlichia species share the host cell receptor and cytoplasmic compartment and identifying the ehrlichial ligand and host cell receptor which are involved in ehrlichial internalization into the unique early endosome. Aim 3 also addresses hypothesis 1 by evaluating the inhibitory effects of the Fab fragment of anti-ehrlichial adhesin (recombinant or natural) IgG or anti-ehrlichial receptor IgG, the recombinant OMP or recombinant receptor protein on ehrlichial binding and internalization. Aim 4 tests hypothesis 2 by examining the role of ehrlichial receptor in recruiting Tf-TfR endosomes and the role of TfR in the ehrlichial inclusion avoidance of lysosomal fusion. Aim 5 addresses hypothesis 3 by examining the effects of immunization with the ehrlichial OMP in a mouse model of human ehrlichiosis.